The relative roles of diffusion and uptake in clearing synaptically released glutamate change during early postnatal development.

Glutamate uptake by transporters expressed in astrocytes combines with synaptic structure to regulate the time that synaptically released glutamate remains in the extracellular space and, consequently, the duration and location of postsynaptic receptor activation. Both factors change greatly in the rodent hippocampus during the second postnatal week when most synapses become established and begin to mature, processes that are influenced by synaptically released glutamate. Transporter expression increases, potentially speeding removal of synaptically released glutamate, whereas extracellular space decreases, thereby slowing dilution. We investigated whether these competing changes influence the glutamate concentration time course and postsynaptic responses in the CA1 region of the mouse hippocampus during this critical period of synaptic development. Our results suggest that the glutamate concentration time course remains relatively consistent over this period, although the primary mechanisms regulating glutamate clearance change. Before the second postnatal week, clearance of synaptically released glutamate depends primarily on diffusion into large extracellular spaces, whereas later in development it relies more on increased uptake capacity. Thus, increased transporter expression during this period accompanies structural changes in the neuropil, preserving a relatively consistent glutamate concentration time course and ensuring that postsynaptic receptor activation remains brief and primarily localized to receptors close to release sites.